Dispensing fluidized pulverulent materials



May 30, 1950 R. H. HALL ETAL DISPENSING FLUIDIZED PULVERULENT MATERIALS Filed July 14. 1949 Irlvcnrors RH Holl Wfl/Imaam to excessive attrition or agglomeration.

Patented May 30, 1950 DISPENSING Ronald H. Hall and Dou inigan Falls, Quebec, Shawinigan Chemicals Limited, Montreal, Quebec, Canada, a corporation of Canada FLUIDIZED PULVERULENT MATERIALS glas M. McKim, Shaw- Canada, assignors to the Dominion of Application July 14, 1949, Serial No. 104,742

4 Claims.

'I'his invention relates to the uniform dispensing of extremely nely ample lime and lime hydrate in pulverulent form. Itis the principal object of this invention to provide a process and apparatus for dispensing such materials at a uniform rate from a storage container or other receptacle to other apparatus for handling, treating, or modifying the same.

It has been disclosed by Goebels in U. S. P. 2,192,287, that numerous pulverulent and granular materials can be discharged at a uniform rate from a storage receptacle if the stored material is first suifused with air or other gaseous medium. The process of causing solid material to disperse in a fluid medium and to assume fluid properties has come to "be known as lluidization. It is now known that certain powdery solids cannot be fluidized solely by suifusion with gas. In some cases this inability to undergo fluidization is due In many cases the reason for inability to undergo fluidization is unknown, while in some cases iluidization is impractical because the process produces so much dust of particles liner than 100 microns diameter that there is a dust nuisance and/or a health hazard. The process of this invention is of particular advantage in handling materials which come under the latter classification, but its application is not restricted thereto.

It has been found that pulverulent solid materials may be maintained in a fluidized state, with the addition thereto of volumes of gas insuilicient to cause separation of dust particles larger than about one micron therefrom, by mechanically agitating the solid material while suffusing the gas thereinto. VIn sorne respects the effect produced does not appear to be true iiuidization," and terms such as expanded state and state of quiescence are sometimes applied to material in this condition. However. throughout the speciilcation and claims, the term iiuidization will be used to describe the process of suspending finely divided solids in a flowing gaseous medium.

The invention comprises a process for dispensing pulverulent""'materials from a storage container or other receptacle at a uniform rate, said process comprising charging pulverulent material to a suitable enclosure forming a fiuidizing chamber, continuously suffusing air or other gaseous medium into the pulverulent material, simultaneously mechanically agitating the material to maintain it in a uidized state of uniform density by combined mechanical agitation and suffusion with gas, and continuously discharging divided materials, for exthrough 4suitable means associated with the Y chamber. The invention further comprises apparatus for dispensing pulverulent materials from a storage container or other receptacle at alumform rate, said apparatus comprising means forming an` enclosure from which pulverulent material may be discharged at a uniform rate to a point of utilization, means.for charging the pulverulent material to the enclosure, means for suifusing air or other gaseous medium into the pulverulent material at the bottom of the enclosure, means for mechanically agitating the material suffused with gas to maintain the material in a, uidized state, and means: for discharging the fluidized material from the enclosure.

A particular embodiment of the invention will now be described, with reference to the accompanying diagrammatic drawings in which:

Fig. 1 is a vertical view, partly in section, of an apparatus for practising the invention.

Fig. 2 is a cross-sectional view taken along the line A-A in Fig. 1.

In the drawings, I is a cylindrical fluidizing chamber, into which the material to be fluidized is charged from the simple hopper 2. An assembly of perforated pipe 3, arranged to distribute gas over the bottom of the chamber and connected to the gas supply 4, is used to suiluse the pulverulent material with gas, which finally escapes by vent I2. A series of rotating blades or paddles 5 is provided for the mechanical agitation of the material; they are mounted in pairs, alternately at right angles on a central shaft E, mounted in bearings 'l and 8, and driven by means not shown.

An opening l0 in the wall of the chamber comthe fluidized material at a constant volume rate 65 municates with an inclined screw conveyor 9, the flights of which are shown diagrammatically in Fig. 2 only. The conveyor is driven by a source of power not shown. By virtue of the fluid pressure of the fluidized pulverulent material in the chamber, the material flows uniformly through the opening I0 and is carried by the conveyor to the discharge Il at a uniform rate. The inclination of the screw conveyor enables the conveyor to act as its own seal or valve so that fluidized material does not flow through it merely under pressure of the static head of fluidized material in the chamber.

A short description of the operation of the above described apparatus will serve to illustrate the operation of the process of this invention.

The nely divided pulverulent material which it is desired to dispense uniformly is charged to the iiuidizing chamber I, from storage through the hopper 2. The transferv of pulverulent'material from storage to .the iluidizing chamber must be such as to maintain the 'level in the nuidlzing chamber within a reasonable range as long as it is desired to dispense the material uniformly, but this transfer does not need to be at a uniform rate; indeed it is not possible to transfer pul-l verulent m-aterials at a uniform rate with ordinary mechanical devices, due to uneven packing of the material, and/or to hanging up or bridging which frequently occurs in pulverulent material when it is being worked. The pulverulent material is most conveniently charged to the fluidizing chamber at or near the top of the chamber. Air or other suitable gas is forced under pressure into the chamber at or near the bottom thereof. The gas is distributed over the cross-section of the chamber by admitting it through a porous plate in the bottom of the chamber or through a series of orifices distributed across the chamber at or near the bottom thereof, as shown at 3 in the drawing. Gas escapes from the chamber at the top thereof, for instance through a, vent as at I2, when the chamber is covered; optionally the chamber need not be covered.

The stirring device located centrally in the chamber to provide mechanical agitation for the fluidized material therein is the novel feature of this particular embodiment of the invention. The stirring device illustrated in the drawing and having blades mounted conveniently at regular intervals along the central shaft, provides sufficient mechanical agitation, for the purposes of the invention, when rotating at unexpectedly low speeds; as an examble, a speed of R. P. M. was found suitable to maintain fluidity in one application, described later in this specification.

When the pulverulent material is of such a nature that suifusion with gas alone is not sunlcient to maintain it in the uidized state, the combined actions of suffusing material in the chamber with gas and mechanically agitating the suifused material serve to maintain it in a nuidized or expanded state. The effect of the mechanical agitation may readily be observed by discontinuing the agitation of a mass of pulverulent material that has been fluidized by combined agitation and suil'usion with gas. The pulverulent material will then be observed to settle gradually to form a packed bed of material cleft by a relatively small number of channels through which the gas continues to pass from the bottom to the top of the bed. This effect is generally known as chanellingf Mechanical agitationprevents the formation of such substantially vertical channels, and the suifusion with gas maintains the pulverulent material in the fluidized or expanded state, in which it has a uniform bulk density, iiows readily like an fluid, and does not pac irregularly or hang up. t

When the pulverulent solid is in the iluidized or expanded state, it flows readily and uniformly out of the uidizing chamber in a manner analogous to the flow of any liquid out of any enclosure. Thus the pulverulent material may be discharged from the fluiglizing chamber in any one of numerous ways. The means illustrated in the drawing, which means has been found to be efficient and versatile, comprises withdrawing the pulverulent material in the flights of an inclined screw conveyor. This device provides a constant volume rate of displacement of material from the fiuidized mass. Since the bulk density of the nuidizing material is independent of the depth 4 of solidsin the fluidixing chamber within a 'wide Vrange of depths, it follows" that the weight disl.placement rate is constant.

The discharge of solids by the screw conveyor may be above or below the level of the fluidized material in thechamber, and the rate of discharge may be varied by varying the speed of the screw conveyor.

The uid pressure of a fluidized pulverulent material is constant at any point in a iluidizing chamber provided that therateV of gas feed to the chamber and the level" of fluidized material in the chamber are fairly steady. The level of iluidized material in the chamber must, of course, be maintained by charging pulverulent material thereto as fiuidized material is discharged. A constant iluid pressure in the fluidizing chamber provides a constant feed of pulverulent material of uniform density to any discharging means. The particular advantage of a uniform speed screw conveyor lies in the fact that, for a constant volume displacement of material from the fluidized mass, such as a screw conveyor provides, the weight displacement is constant regardless of the depth of iiuidized material in the chamber, since the bulk density of the fluidized material is independent of the amount of solids in the fluidizing chamber. Thus the apparatus illustrated in the drawing is particularly capable of dispensing pulverulent material at a constant rate, since the level of fluidized material in the chamber may vary over a wider range, than that permissible in apparatus using other discharge means, without affecting the rate of discharge from the chamber.

As an appropriate example of the invention, a

specific apparatus, which has been successfully operated, is hereunder described.

The elements of the apparatus were generally of the type illustrated in the drawing. The iiuidizing chamber was a iii-gauge sheet steel cylindrical tank, six feet high and three feet in diameter, and was mounted on a suitable I-beam base. A metal chute, leading into the top of the tank and requiring manual loading, served as the means for charging pulverulent material to the tank. Air was admitted to the tank via a rotameter, through a coil-shaped sparger made of 1/2 inch standard pipe mounted six inches above the bottom of the tank. Air was dispersed from the sparger through twenty-four 11g inch orifices in the pipe. A paddle agitator, driven by a, 1/2 H.P. motor through a speed reducer at 20 R. P. M., served as the mechanical agitating means. The paddles were 1 inch by A inch strip steel and 'were welded to a 1 inch central shaft, with alternate pairs of paddle on opposite sides of the shaft, mounted at right angles to each other at 6 inch intervals along the shaft. The discharging means on the apparatus was a. single "flight, 4-inch pitch, screw conveyor mounted in a small duct on the side of, and open to, the tank. The conveyor was 1l feet long and mounted at an angle of 30 degrees to the horizontal. It was driven at the lower end through a set of right-angle bevel gears connected by a chain and Y sprocket to a variable speed drive.

asomar ing quantities of hydrate down the charging chute, and lime hydrate was discharged `from the tank at various rates by the screw conveyor. variations in the rate of discharge being obtained byrvarying either the rate of turning the screw conveyor or the rate of admission of air through ing means might also be a group ol slow speed stirrers, mounted on individual axes of rotation, turning in cooperation to prevent channelling and maintain fiuidization of the bed.

the sparger. Suusion of the lime hydrate with s air was found to be inadequate to uidize hydrate in the tank. The combination oi suilusion with air and agitation by the paddle agitator rotating at about 20 R. P. M. was found to maintain hydrate in the tank in a uidized or expanded 't state, in which state it could be uniformly discharged from the tank. The iluidized bed of hy drate had a depth of from five feet to sii: feet, usually close to live and one half feet.

A number of runs were made in which the level oi nuidized hydrate was maintained by manual additions to the tank as air was admitted to the tank at e. rate of 100 ou. It./min. and hydrate discharged. The discharge rate of hydrate irom the tank was determined with the screw con- Y' veyor turning at various speeds. The following table gives the discharge rates, in pounds per minute, obtained with a series oi conveyor speeds.

Conveyor Discharge pees new R. P. M. ltd/mm.

27 a7 as. s so a9. a t2. t 4e. s 62. s 64 7o es s2. t

The importance of these runs lies in the fact I' that it was possible to maintain each of the above ior an indednite period.

From the preceding descriptions it will be apparent that numerous modlcations may be made in the specic expediente described. For instance, the means for uniformly discharging uidiaed material from a uidizing chamber could he a .iet oi air or other gas directed into s, discharge conduit associated with the iluidizlng chamber, as disclosed in the aforementioned Goebels patent. The means for mechanically agitatina a pulverulent material could be one or more high-speed propellers turning in the duid. ized bed. Such propellers would reduire more power than a slow-turning agitator. but would enable smaller volumes of gas to be used for amusing the materiai. The mechanlcl @Eltern .of pulverulent lime hydrate to a pressing or briquetting machine. Such machines operating at constant speed require a constant feed of pulverulent lime hydrate to produce briquets of uniform density. Other advantages will be apparent to those skilled in the art.

What we claim is:

i. Apparatus for uniformly dispensing a pulverulent solid material comprising (l) means forming an enclosure from which the pulverulent material may be discharged at a uniform rate, (2) means for charging pulverulent mate rial to the enclosure, (3) means for sudusing a gaseous mediuminto the pulverulent material at the bottom of the enclosure, (4i) means for mechanically agitating the material suused with gas to maintain the material in a uidized state, (5) and screw conveyor means ior dis-a charging the fiuidized material from the enclosure. said conveyer means being in open contact with fluidized material in the enclosure above the means for suiiusing the gaseous medium.

d. Apparatusas claimed in claim 1, in which the means for mechanically agitating the pulverulent material is a stirring device.

3. Apparatus as claim in claim l, in which the means for mechanically agitating the pulverulent material is a set of stirrers turning on individual axes of rotation.

4. Apparatus. as claimed in claim l, in which the screw conveyor means is arranged to elevate and discharge material from the enclosure at a level above the level of nuidized material in the enclosure.

' RONALD H. HALL.

DOUGLAS M. MC'KM.

REFERENCES CITED The following references are oi record in the ille oi this :patent:

UNITED STATES PATENTS Number Name Date 525,095 Detwiler Aug. 28, 1894 1,616,547 Pontoppidan Feb. 8. 1927 1,839,456 Anderson Jan. 5, 1932 

